Teslas, solar power and the electric grid (Howard Hayden) USofA

From Energy: A Textbook:

Liquids, such as gasoline, diesel fuel, and ethanol, are measured and sold by volume, usually the gallon (US) or the liter (the rest of the world). Gases are measured and sold by their volumes at atmospheric pressure (cubic feet or cubic meters). It is therefore nice to have a table of the volumetric heat contents. The values are given in Table A12.

Note that one barrel (abbrev. bbl) is 42 gallons, and is not to be confused with its larger cousin, the drum, which is 55 gallons.

Let us use one of those figures to do a calculation about automobiles. Suppose that you have a car that gets 30 miles per gallon of gasoline, and suppose that you’re travelling at 60 miles per hour. What is the fuel input power? We start with understanding that the energy is in the gasoline, and the time is the hour. In one hour, we go sixty miles, and burn two gallons.

Fuel Input Power

=

2 gallons x 1 hour x 0.131x109 J x W s hour

Automobile:

= 72,800 W

3600 s

gallon

1J

(15)

In words, your car is using as much power as 728 100-watt incandescent light bulbs.

(Europeans: The heat content of gasoline is 34.5 MJ/liter)

From The Energy Advocate July 2017:

Exposing Science Secrets!

If you use your search engine to find “Tesla plans to disconnect from the grid," you will find a great number of regurgitations of an article (whose origin I did not try hard to find) about how Elon Musk plans to use solar energy to run its supercharging stations. There are twelve 120-kW stations in a facility.

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Writing in a forum of the Society of Automotive Engineers (SAE), Stan Jakuba fairly well shredded the idea of powering a dozen Superchargers with solar panels. He calculated the energy and power requirements to charge up a dead Tesla 324-MJ battery in an hour, adopted an extremely generous figure (40%!) for the photovoltaic collection efficiency, and came up with this statement: So for a 12-station charging facility we would need 12,000 m2 of panels—one and a half soccer fields or 1 and a half footprints of the Empire State building…

There is no need to go over the calculations here (See TEA, June 2017). What was really amazing was this comment on Jakuba’s posting on the SAE website: Why was not this written a generation ago?

A generation ago, nobody considered Supercharging stations for battery-powered cars, the idea of which had been abandoned well over a century ago. The problems in 1900 were two: the batteries and the charging system. The problems in 2017 are two: the batteries and the charging system.

There is nothing new in the idea that energy required to charge a battery is the voltage times the current times time. That information can be gleaned from any elementary physics or electrical engineering textbook. Most of those books say little or nothing about the intensity of sunlight, or about PV-cell efficiency, but the information can be had readily on the Internet. In desperation, one might consider a trip to the library. In any case, it is gratifying to see a glint of understanding in somebody’s eyes.

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From The Energy Advocate June, 2017:

The Tesla At the website www.tesla.com/charging, you can read the charging specifications for two Tesla models. I will use the data for the Model S, which differs only slightly from Model X. With a “standard wall connector” (240 volts, 40 amperes; no data are given for 120volt, 20-A standard wall connectors), you can charge your Tesla for 45 miles of driving in a mere 1 hour and 34 minutes. Or, with two hours and 54 minutes, you can charge enough for 100 miles (160 km).

These numbers, however, don’t quite tell the story. For example, if you’re getting the power from a 240-V line on a 40-ampere circuit breaker, you are probably not drawing 40 amperes of current. The Tesla website, however, shows that you can plug into a 120kW Tesla supercharging station, and get enough charge for a 100-mile trip in 15 minutes. This means that the energy produced by the charging station is 40 kWh. That is, it takes 0.4 kWh (1.44 MJ) for the car to travel 1 mile. The US average efficiency in producing electricity (net generation) is 35%, so this 1.44 MJ of electricity is equivalent to a heat input of 4.11 MJ. By way of comparison, a car that gets 30 miles per gallon of gasoline (as the sleek Tesla surely would if it used gasoline) uses 1/30th of a gallon (at 131 MJ/gallon), or 4.36 MJ to 3

go a mile. To be perfectly clear on this point, battery-powered cars save precious little energy.

It takes 15 minutes with a 120-kW Tesla charging system to provide for 100 miles (160 km) of travel, from [3].

Dreamers, of course, imagine that we can charge electric vehicles from solar panels.

More to the point is whether our present electrical system can handle the load if there are suddenly lots of new battery-powered electric vehicles (BEVs). Actually, there are two separate considerations. The first is whether we have enough power generation, and the second is whether our distribution system can deliver the electricity.

In the US, the total generating capacity is about 1,000 GW, and the year-round average consumption is about 465 GW. In principle, therefore, we could charge cars when power consumption is otherwise low, making use of the unused capacity.

Donn Dears writes [4] It would appear that there could be 87 million BEVs on the road if spread uniformly across the country, before new power plants would have to be built to accommodate the additional load caused by charging the batteries of BEVs.

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Power generated in power stations is immediately converted to high voltage (numerous voltages between 138 kV and 765 kV), depending on system) for transmission. Substations use transformers to reduce the voltage to 69 kV or 26 kV for distribution, and (for homes) other transformers (such as the pad-mounted and pole-mounted ones shown to the right) reduce the voltage to the 120V/240V system. Herein lies the problem; the local transformers are inadequate to handle any more than the occasional BEV. A little background is in order for non-experts. Motors, electric lights, computers, and the like consume power, measured in watts. For AC, it’s not quite as simple as Power = voltage times current, (with units: watts = volts times amperes) but for our purposes that will do. By contrast, a transformer consumes very little power (becoming heat) as it converts one voltage to another. The general rule is that the product of the voltage and current on the primary side is equal to the product of voltage and current on the secondary side: the power input = the power output. (Well, the power output is a little less because of the heating of the transformer.)

There are two things the cause heating of the transformer: the resistance in the wires and some losses in the iron core as the magnetic field flops back and forth. In any case, a transformer is designed to transform some maximum product of voltage and current, and is rated for so-many kilovolt-amperes (kVA).

The typical pad-mounted transformer is rated at 50 kVA, enough for four homes. Dears says [4]: It might only take one homeowner adding a BEV using the simplest, lowest voltage and amperage charging system to overload an existing 50 KVA transformer.

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If all four homeowners add a BEV and the existing 50 KVA transformer is 75% loaded, it could easily require that the 50 KVA unit be replaced with a 100 KVA transformer.

In other words, the first effect that BEVs will have on the electrical system will be on the local distribution system.

[3]

https://www.tesla.com/charging

[4] Donn Dears, “Charging a battery-powered vehicle (BEV) starts outside the home at the distribution transformer,� 5 May 2017 http://www.powerforusa.com/2017/05/05/bev-charging-basics/

Cheers, Cork ________________________ Howard "Cork" Hayden corkhayden@comcast.net NEW! Energy: A Textbook, $25 at www.energyadvocate.com and www.valeslake.com P.O. Box 7609 Pueblo West, CO 81007

Consensus is the enemy of science July 2017

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